The present invention relates to a method and apparatus for communications data transmission using wavelength division multiplexing (WDM). More specifically, the present invention relates to a method and apparatus for enabling hyperfine wavelength division multiplexing (HWDM) by subchannelizing each channel of conventional dense wavelength division multiplexing (DWDM) into many subchannels.
Fiber optic cable is widely used for data transmission and other telecommunication applications. However, the relatively high cost of installing new fiber optic cable presents a barrier to increased carrying capacity.
Wavelength division multiplexing (WDM) enables different wavelengths to be carried over a common fiber optic waveguide. WDM can separate the fiber bandwidth into multiple discrete channels through a technique referred to as dense wavelength division multiplexing (DWDM). This provides a relatively low cost method of substantially increasing long-haul telecommunication capacity over existing fiber optic transmission lines.
Techniques and devices are required, however, for multiplexing the different discrete carrier wavelengths. That is, the individual optical signals must be combined onto a common fiber optic waveguide and then later separated again into the individual signals or channels at the opposite end of the fiber optic cable. Thus, the ability to effectively combine and then separate individual wavelengths (or wavelength bands) from a broad spectral source is of growing importance to the fiber optic telecommunications field and other fields employing optical instruments.
Devices that assemble multiple tightly spaced carrier wavelengths within a single fiber are called multiplexers. Devices that separate the carrier wavelengths at the receiving end of a fiber are called demultiplexers or channelizers. The following types of known technologies can be used as WDM channelizers.
The Fabry-Perot interferometer is a known device for resolving light into its component frequencies, or equivalently, its component wavelengths. FIG. 1 illustrates one example of a prior art Fabry-Perot interferometer. The illustrated device comprises two mirrors M1 and M2. Each of the two mirrors M1 and M2 is a partially reflecting mirror. The mirrors M1 and M2 are separated by an air space. Alternatively, the Fabry-Perot interferometer device could be made by coating both sides of a transparent plate with a partially reflecting material.
Light from a spectrally broadband source is input at plane S1. Light rays at an angle xcex8 and a wavelength xcex undergo multiple reflections between mirrors M1 and M2. The light rays interfere constructively along a circular locus P2 in the output plane S2. The condition for constructive interference that relates a particular angle xcex8 and a particular wavelength xcex is given by
2dcosxcex8=mxcex
where d is the separation of the partially reflecting surfaces, and m is an integer known as the order parameter. The Fabry-Perot interferometer thereby separates the component frequencies of the input light by using multiple beam reflection and interference. It is apparent from the equation above that the output light pattern of the system, i.e., the interference fringes, in the case of a diverging input beam, is a set of concentric circular rings. One ring is present for each wavelength component of the input light for each integer m, with the diameter of each ring being proportional to the corresponding light frequency.
The Fabry-Perot interferometer is not well-suited for use as a WDM channelizer due to the difficulty in obtaining high optical throughput efficiency. If the input beam is divergent, e.g., the direct output of an optical fiber, then the output pattern for a given wavelength is a set of rings. Multiple wavelengths produce nested sets of concentric rings. It is difficult to collect this light efficiently and concentrate it at multiple detector points, or couple it to multiple output fibers, especially while maintaining the separation of wavelength components that the interferometer has produced. If the input beam is collimated, e.g., the collimated output of an optical fiber, then the beam can be fanned over a narrow range of angles to produce only a single-order output (e.g., m =+1) for each wavelength of interest. This fanning makes it easy to concentrate the output light at multiple detector points or fibers, but there is inherently high loss. The throughput efficiency can be no greater than 1/N, where N is the number of wavelength components to be separated.
FIG. 2 illustrates an example of a Lummer-Gehrcke interferometer. The illustrated interferometer comprises an uncoated glass plate and a prism for coupling a beam of light into the plate. Internally, the plate is highly reflective at internal incidence angles that approach the critical angle. The internal incidence angle controls the reflectivity of the surfaces. The output of the illustrated Lummer-Gehrcke interferometer is a series of multiple reflected beams that have a frequency-dependent phase shift from beam to beam and that are focused at the output plane by a lens. The interference fringes that are formed at the output plane in the case of a diverging input beam and a particular wavelength xcex are a family of hyperbolae near the center of the output plane. Each wavelength component of the input beam gives rise to a unique set of hyperbolic fringes.
The Lummer-Gehrcke interferometer relies upon a glass plate that is uncoated. However, the absence of a surface coating means that it is not possible to tailor the fringe intensity profile. This makes the Lummer-Gehrcke interferometer impractical for use in WDM applications in which the fringe profile controls the channel filter shape.
The Lummer-Gehrcke interferometer also is not well-suited for use as a WDM channelizer due to the difficulty in obtaining high optical throughput efficiency. If the input beam is divergent, e.g., the direct output of an optical fiber, then the output pattern for a given wavelength is a family of hyperbolae. Multiple wavelengths produce nested sets of hyperbolae. It is difficult to collect this light efficiently and concentrate it at multiple detector points, or couple it to multiple output fibers, especially while maintaining the separation of wavelength components that the interferometer has produced. If the input beam is collimated, e.g., the collimated output of an optical fiber, then the output pattern for a given wavelength is a set of focused spots corresponding to multiple interference orders. Again, it is difficult to collect this light efficiently, and there is generally an inherent loss. The throughput efficiency can be no greater than 1/N, where N is the number of focused spots per wavelength.
FIG. 3 illustrates an example of a Virtually Imaged Phased Array (commonly referred to as a VIPA). The VIPA illustrated in FIG. 3 includes a rectangular glass plate 10 that has a 100% reflective coating 12 on a first side and a partially reflective coating 14 on an opposing side. Light enters the plate 10 below the reflective coating 12 in the form of a focused line source 16 produced by cylinder lens 18.
FIG. 4 illustrates an operational side view of the VIPA. Input light rays 20 and 22 represent the boundaries of the line-focused input beam. The lens 18 focuses the input rays at the point 24, after which the rays diverge as the beam propagates. The focused input rays 20 and 22 are partially reflected by the coating 14 and then totally reflected by the coating 12. This reflection produces a virtual image of point 24 at location 26. The reflective process is continued, producing additional receding virtual images at locations 28 and 30. This process produces virtual images of the line source that recede away from the input side of the glass plate (i.e., to the left of element 10 in FIG. 3) and that are distributed in the y direction.
FIG. 5 illustrates the optical distribution of the diverging light beams at the exit surface of the glass plate. The numbered circles 32, 34 and 36 are intended to call the reader""s attention to the areas of interest on the coated surface 14. The circles represent the size of the light beams exiting the plate. The line focused input is illustrated at point 24, the twice reflected light that has diverged due to propagation is illustrated at circle 32, the four-times reflected light that has diverged even more is illustrated at circle 34, and the six-times reflected light that has diverged even more is illustrated at circle 36. In the example illustrated in FIG. 5, after more than six reflections the diverging light beams overlap and blend into an interference pattern.
As shown in FIG. 5, each of the successive beams 32, 34 and 36 that exits the surface 14 of the VIPA plate 10 appears to originate from the line source images 26, 28 and 30, respectively, as shown in FIG. 4. The light from these images diverges as the light propagates inside the glass plate 10. Part of the light from each image exits the plate through the partially reflective coating 14. The exiting beams interfere with each other. The interference pattern is collected by a lens 38 and focused onto a detector array 40 (FIG. 3).
In the illustrated VIPA the beams diverge and overlap at the partially reflective surface 14. This overlap effect makes weighting the individual virtual sources possible only in an area-average sense, thus limiting the degree to which the channel filter shape can be tailored.
The VIPA requires a line-focused input. The line-focused input means that the VIPA may provide a relatively compact device for coarse channelization (i.e., wide channel spacing on the order of one hundred GHz). However, the line-focused input makes the VIPA impractical for fine channelization (i.e., narrow channel spacing on the order of one GHz) due to the fact that a thicker plate is needed, which would result in excessive beam divergence and overlap at the exit surface.
Thin film interference filters require a different coating design to separate each wavelength component of an input beam. Since the interference filters produced by thin film coatings tend to have relatively wide passbands, they cannot achieve high resolution (twenty five GHz or finer). These limitations essentially preclude the use of interference filters as viable hyperfine wavelength division multiplexing (HWDM) channelizers.
In the classical diffraction grating, as used for WDM channelization applications, the dispersive element is a grating embedded in a monoblock of silica. The input optical fibers may be directly fixed to the block. The grating may comprise several tens of grooves to several thousands of grooves per millimeter. The grooves may be fabricated, for example, by etching with a diamond tool or by holographic photo-exposure. The grating diffracts incident light in a direction related to the wavelength of the light, the groove spacing, and the incidence angle. Consequently, an incident beam comprising several wavelength components will be angularly separated into different directions. Conversely, a reverse mode of operation is possible in which several beams of different wavelengths coming from different directions may be combined (multiplexed) into the same output direction.
Diffraction gratings of reasonable size do not have sufficient resolution for HWDM. For example, for a channel separation of 1 GHz, a grating would have to be significantly longer than twelve inches to achieve an adequate channel filter shape. They also have high optical insertion loss, making them inefficient for use in high resolution WDM systems.
Diffraction gratings tend to produce undesirable polarization effects. The diffraction efficiency depends on the polarization of the incident beam. For a given wavelength, this effect increases when the groove spacing decreases. Typically this effect is small when the groove spacing is at least ten times larger than the wavelength, but the effect becomes important when the groove spacing is reduced to a few wavelengths in order to achieve higher angular dispersions.
The arrayed waveguide grating (AWG) is an integrated-optic passive delay line device that performs channelization. The AWG is designed to increase the resolving power, i.e., the fine splitting of the wavelengths, over that obtainable with classical diffraction gratings. AWG""s were first proposed around 1990 by Takahashi and others in Japan and by Dragone and others in the U.S. AWG""s increase the optical path difference between the diffracting elements by using a waveguide structure equivalent to the well-known Michelson echelon gratings in classical optics. AWG""s have the inherent disadvantage of a much smaller free spectral range that limits the total number of channels and increases the near-end crosstalk that affects bidirectionality. It is difficult to achieve resolution better than fifty GHz using an AWG. AWG devices capable of one GHz resolution would be physically large, expensive, and have very high loss.
A fiber grating is made by recording a Bragg grating in the core of a single-mode fiber that is made photosensitive by doping it with, for example, germanium. This grating may be used as a narrowband filter. It is necessary to use one grating per wavelength channel, so there is an inherent limitation on the number of channels that can be demultiplexed with such devices due to the shear bulk of the resulting system. A primary disadvantage of a fiber grating is that it can reflect only one wavelength. An N channel system therefore requires N fiber gratings.
The present invention provides a method and device for independently operating on each of one or more optical inputs and producing spatially separated independent optical beams at the output. The spatial separation among the output beams is a function of the frequency components of the corresponding optical input beams. The invention enables the simultaneous channelization of hundreds of discrete input beams into their constituent frequency components.